Foam sheet materials have been used, for example, as seals and gaskets (sealing materials for fixing) in OA devices, computer-related devices, small communication devices such as cellular phones and small music-regenerating devices and others that demand waterproofness, air tightness, shock-absorbing properties and others. For example, known is a method of providing foams, such as open-cell polyurethane foams, with waterproofness by decreasing air permeability thereof by means of reducing the size of the cells therein and also adding a water-repellent hydrophobic additive (such as paraffin, wax, coal tar, asphalt or the like) thereto (see Patent Document 1). Also known is, for example, a rubber-based vulcanizing and foaming composition, specifically a mixture containing EPDM, a vulcanizing agent, a blowing agent, and a softener, that is useful in production of a low-density foam superior in water-cutoff efficiency and yet flexible enough for favorable operation (see Patent Document 2).
However in the case of the method of adding a hydrophobic additive to the foam (see Patent Document 1), the resin component and the hydrophobic component melt less easily, occasionally resulting in unevenness in cell diameter or nonuniformity of cell distribution and thus, causing a problem that it was not possible to obtain sufficient waterproofness because of fluctuation in water-cutoff efficiency. It was also difficult by the method above to achieve desired waterproofness only with the foam itself, because the average cell diameter of the waterproofing foam is higher at 100 μm or more, the surface area smaller and the air permeability higher.
In addition, it was occasionally difficult to obtain sufficiently high water-cutoff efficiency with the rubber-based vulcanizing and foaming composition containing EPDM, a vulcanizing agent, a blowing agent and a softener (see Patent Document 2), because the average cell diameter of the cells in the rubber-based foam obtained is higher at 1.0 to 1.5 mm, the surface area smaller and the air permeability higher.
Further, the clearance (opening) for such a waterproofing material in image display devices and optical members such as cameras and lenses, in which conventional waterproofing materials had been used, was sufficiently larger. Thus, these waterproofing materials could be used without intensive compression. For that reason, there was no particular need for attention to the compressive repulsion force of the waterproofing material.
However, products in which optical members are installed are made increasingly smaller in thickness recently and thus, the clearance of the region where the waterproofing material is used is seemingly reduced similarly. Accordingly, there exists a demand for a waterproofing material superior in waterproofness and also in flexibility that can be compatible with minute clearance.    Patent Document 1: Japanese Unexamined Patent Application No. 55-71777    Patent Document 2: Japanese Unexamined Patent Application No. 2002-146072